Photographic emulsion silver gage

ABSTRACT

Apparatus measuring the silver content in photographic film emulsion utilizing an Americium-241 radioisotope source causing fluorescent X-ray emissions from the emulsion silver, and electronic pulse height analysis with compensation for radiation attributable to other constituents of the film than silver.

United States Patent Severance [54] PHOTOGRAPHIC EMULSION SILVER GAGE [72] Inventor: William A. N. Severance, Newark,

Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: May 20, 1970 [21] Appl. N0.: 38,929

[52] US. Cl ..250/51.5, 250/833 D [51] Int. Cl. ..G01n 23/22 [58] Field of Search ..250/51.5, 83.3 D

[56] References Cited UNITED STATES PATENTS 2,897,367 7/1959 Anderrnann et al ..250/5l.5 2,675,482 4/1954 Brunton ..250/83.3 2,449,066 9/ 1948 Friedman ..250/51.5 2,928,944 3/ 1960 Reiffel ..250/5 1 .5 2,947,871 8/1960 Friedman ..250/5 1.5

[ 1 Aug. 29, 1972 FOREIGN PATENTS OR APPLICATIONS 920,356 3/1963 Great Britain ..250/83.3 960,373 6/1963 Great Britain ..250/83.3 598,814 l/1961 Belgium OTHER PUBLICATIONS Alloy Analysis and Coating Weight Determination Using Gamma-Ray Excited Souces by J .A. Hope et a1 From International Journal of Applied Radiation and Isotopes, Vol. 16, 1965, pgs. 9- 14.

Primary Examiner-William F. Lindquist Attorney-Harry J. McCauley [57] ABSTRACT Apparatus measuring the silver content in photographic film emulsion utilizing an Americium-24l radioisotope source causing fluorescent Xray emissions from the emulsion silver, and electronic pulse height analysis with compensation for radiation attributable to other constituents of the film than silver.

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11V VEN T 012 W ZZUdm/AJV. Severance HM 9. w 0M1 /I 'I'TORNF Y PI-IOTOGRAPIIIC EMULSION SILVER GAGE BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises apparatus for measuring the silver content in the silver emulsion coating of photographic film comprising an Americium-24l radioisotope source, means directing the radiation from the source at close range and at a composite angle reducing scatter with respect to photographic film to be tested, detector means generating electrical signals responsive to electromagnetic radiation (photons) received from the film in the energy region (A), displaying the 24.9 Kev maximum, primarily characteristic of silver fluorescence but inclusive also of some scatter contributed by other components of the film, and also in the energy region (B), displaying the 59.6 Kev maxima, constituting scatter contributed by all components in the film, and electronic pulse height analyzer means receiving the output of the detector means determining the silver content of the photographic film as a function of the difference between the total of the (A) radiation measured and a preselected quantity of the (B) radiation substantially equivalent to the fraction of the (A) radiation measured attributable to scatter from other components of the photographic film than the silver content.

DRAWINGS FIG. 1 is a cross-sectional partially schematic view of reference channel intensity for the apparatus of FIGS.

1 and 3 demonstrating the combined equivalent compensation for scatter contributed by film base and water, and

FIGS. 6A and 6B are recorder read-outs for the same specimen of commercial X-ray film wherein FIG. 6A is that of the wet emulsion product whereas FIG. 6B is that of the dry emulsion product.

It is desirable, in the manufacture of photographic film, to,-continuously measure the silver content in the wet photographic emulsion as soon as practicable after application to the film base in order to maintain good quality control of the final product as well as to conserve the use of high-cost silver.

Photographic film is, of course, radiation-sensitive and it is paramount that inspection radiation not expose or fog the product. Also, different radiation sources give different film radiation outputs, scatter or other effects for the different components of the coated film which interfere with accurate silver gauging and sources, in the number of two, three, or four individual sources 10 of, e.g., 50 millicurie strength Americium- 241 each, disposed at equi-radial distances and equiangular spacings from the longitudinal axis of detector 11 in order to distribute the radiation incident on the photographic film uniformly over a large area such that the radiation intensity per unit area of film will be small for each photon detected. Moreover, such a source disposition insures more representative readings. The construction can conveniently embody a unitary head design, indicated generally at 15, wherein sources 10 are supported on the ends of radial spokes 15a, only two of which are detailed in FIG. 1.

Sources 10 are disposed close to the plane 12 of photographic film travel at a distance, typically, of 1.70 inches directed inwardly and collimated at an acute angle of radiation incidence a with the film, preferably of about 45, in order to reduce radiation scatter to a minimum. Since sources 10 have an appreciable transverse width, it will be understood that radiation from given points of each source impinge on the photographic film at a great variety of angles. Nevertheless, with the film-detector ll angle substantially normal, it has been found that the composite angle of radiation impinging on the film with the source axis-film angle a about 45 and source-to-film distance approximately equal to film-to-detector distance measured along an angle of reflection equaling a, is such as to reduce scatter to a practical minimum. The signal-to-noise ratio reduction occasioned by such scatter has not hitherto been viewed as an important factor contributing to inaccuracy in radiation gaging; however, I have found it very desirable to minimize scatter insofar as practicable.

For the configuration described, the placement of detector 11 is also relatively critical, as is shown by FIG. 2. Here the optimum detector-to-film spacing for maximum radiation intensity preservation is approximately 1.25 inches, although dispositions in the range of 1 5/32 to l 11/32 inches can be tolerated without changing the average reading by more than $0.5 per cent. The difference between these limiting values is three-sixteenths inch, equal to 194 mils. Conventional film support devices are capable of supporting running films with lateral film displacements not exceeding about :50 mils, so that lateral film displacement constitutes no obstacle to overall measurement precision.

Americium-241 is a unique source for the purposes contemplated, since it emits 26.4 Kev gamma rays which are nearly ideal for producing silver fluorescence. In addition, it also emits 59.6 Kev gamma rays, which are less ideal in energy but greater in number per unit time and, so, also useful for producing fluorescent X-rays. As hereinbefore mentioned, 50 millicurie sources 10 are suitable, these preferably being backed with compacted antimony powder in order to convert some of the 59.6 Kev gamma rays normally lost into useful 30 Kev X-rays.

In accordance with standard practice, the sources 10 are preferably doubly encapsulated, with each capsule sealed with a 5 mil stainless steel window 10a.

Gauging head 15 is fabricated from stainless steel and is, as hereinbefore described, designed to provide approximately collimation between the sources and detector 1 l in order to minimize radiation scatter.

Under the conditions described, the radiation-exposed areas on the film are non-overlapping ellipses with axes of approximately 1 inch and 1.5 inches.

Detector 11 can be a 2 inches diameter thin cleaved crystal of sodium iodide optically coupled to a low noise photomultiplier tube and hermetically sealed in an aluminum container provided with a window disposed toward the film 12 fabricated from 5 mil beryllium foil.

Detector 11 is optically coupled to a low noise photomultiplier tube (not detailed) and the voltage pulse output of the latter is supplied via electrical connection 16 as input to preamplifier 19 (FIG. 3) which, in turn, feeds its output to amplifier 20. Amplifier 20 supplies voltage pulse signals to two separate channels, the lowermost of which, as seen in FIG. 3, is the silver channel and the uppermost of which is the reference channel.

The silver channel comprises a conventional pulse height analyzer 21 and a conventional ratemeter 22 set to count and display the photons in the energy region arbitrarily denoted (A), FIG. 4, which is primarily characteristic of silver, the peak of which is at approximately 24.9 Kev. Similarly, the reference channel comprises a conventional pulse height analyzer 23, incorporating means for dynamic feedback stabilization applicable to the gauging head voltage source 24, together with its ratemeter 25. The reference channel is reserved to the display of photons in the energy region arbitrarily denoted (B), FIG. 4, the peak of which is at 59.6 Kev, including the scatter contributed by silver.

The outputs of ratemeters 22 and 25 are passed to difference amplifier 26, and the voltage output of the latter is supplied to the silver content recorder 27.

The apparatus described lacks discrimination as regards particular mono-energetic inputs, the existing situation being that shown generally in FIG. 4.

Here it will be seen from curve D, which is the test spectrum for X-ray film coated with dried silver-containing emulsion over the energy range of 80 Kev, that the curve of intensity variation plotted on the ordinate axis displays three separate peaks. The first, for bromine fluorescence, primarily, has a peak maximum of about 13.2 Kev, the second for silver fluorescence, primarily, but with some contribution from radiation scattered by underlying film base and emulsion materials, is at about 24.9 Kev, and the third, for radiation scattered from film base and emulsion materials primarily, is at about 59.6 Kev. If the identical film base, free of all emulsion, is tested under identical conditions, it displays two energy peaks, one being at about 26.4 Kev, whereas the other is at 59.6 Kev, as shown approximately by plot E, FIG. 4, which peaks are due to scattering of the Americium 241 radiation by the base film.

The first of the latter peaks intrudes on the silver content characteristic radiation (A). The silver intensity value of curve D must be compensated for the contribution of scatter if silver is to be determined with accuracy.

This is accomplished by examining a length of uncoated film base, i.e., one entirely free of emulsion and silver, and adjusting the response of the reference channel, by appropriate manipulation of its potentiometer, to exactly counterbalance the response of the silver channel. At this point recorder 27 reads zero silver, which is entirely accurate.

Then, if emulsion-coated film is fed through the gauge, subsequent readings will all be in terms of silver content exclusively. The reason for this is, as can be seen by inspection of FIG. 5 hereinafter described, that there is an invariant proportionality, at least over the region of analytical interest, between the scatter caused by emulsion, film base and other components, exclusive of silver and halogens, however, for the characteristic radiations of (A) and (B). Thus, for any given thickness of film, a correction of radiation sensed in the reference channel based on the known proportionality relationship accurately compensates the silver channel so that the determination of the apparatus is exclusively in terms of silver.

I have ascertained that any water in the emulsion behaves indistinguishable from film base, so that compensation for the latter automatically effects simultaneous compensation for the water as well.

The graphical proof of equivalence of water to film base is shown in FIG. 5, wherein the variation of silver channel intensity, as ordinate, is shown versus reference channel intensity as abscissa, drawn by a conventional X Y plotter.

Here a plurality of thin sections of dry film base, each free of emulsion and silver, were superposed one on the other, so that a reading for 18 mils total thickness was obtained for the upper right-hand composite thickness indicated by the 18 mil arrow. Then a piece of film 3 mils thick was removed and the trace dropped along the sloped line to the 15 mil arrow point. Successive removals of the next piece of 4 mil film base dropped the response to l l mils, and of yet another 4 mil thick piece dropped the response to 7 mils. A final 3 mil piece lowered the response to the 4 mil point denoted. Finally, the last piece of film, measuring4 mils thick, was removed, which gave the left-hand value denoted AIR in FIG. 5.

Now, the 4 mil strip was again placed under the gauge and slightly bowed upwardly at the edges to retain water, which was poured on until a reading near the 18 mil arrow head was obtained. After this, water was removed by sponging it off in successive steps using paper tissue, during which it was found that the gauge response obtained proceeded step-wise from right to left down the plot line of FIG. 5, as indicated by the several plot points, demonstrating that the effect of water was, for all practical purposes, exactly equivalent to that of film base.

This was corroborated by an actual plant test wherein the gauge of this invention was utilized to determine silver in product coated with wet emulsion (FIG. 6-A), followed by the same determination repeated for the dried product, which gave the indication of FIG. 6-B. From this, it is seen that the presence of water is compensated by the gauge and causes little or no inaccuracy in silver content determination.

It will be understood that digital circuitry can be readily substituted in known manner for the ratemeters and analog circuitry of the apparatus embodiment described.

What is claimed is:

1. Apparatus for measuring the silver content in the silver emulsion coating of photographic film comprising, in combination, an Americium-24l radioisotope source, means directing the radiation from said source at close range and at a composite angle reducing scatter with respect to photographic film to be tested, detector means generating electrical signals responsive to electromagnetic radiation received from said film in the energy region (A), displaying the 24.9 Kev maximum, primarily characteristic of silver fluorescence but inclusive also of some scatter contributed by other components of said film, and also in the energy region (B), displaying the 59.6 Kev maxima, constituting scatter contributed by all components in said fihn, and electronic pulse height analyzer means receiving the output of said detector means determining the silver content of said photographic film as a function of the difference between the total of said (A) radiation measured and a preselected quantity of said (B) radiation substantially equivalent to the fraction of said (A) radiation measured attributable to scatter from other components of said photographic film than said silver content.

2. The apparatus of claim 1 in which the longitudinal axis of said source and its associated collimator is disposed at an angle of approximately 45 with respect to said photographic film to be tested at an inclination towards said detector means and the longitudinal axis of said detector means is substantially normal to said photographic film, with the face of said detector means substantially equidistant from said photographic film as said source, measured from the point at which said longitudinal axis of said source intersects said photographic film.

3. The apparatus of claim 1 wherein said electronic pulse height analyzer means comprises individual pulse height analyzers with associated ratemeters reserved to measurement of said (A) radiation and said (B) radiation individually, adjusting means regulative of the output signal from the (B) radiation measuring one of said associated ratemeters, a diflerence amplifier receiving 

2. The apparatus of claim 1 in which the longitudinal axis of said source and its assoCiated collimator is disposed at an angle of approximately 45* with respect to said photographic film to be tested at an inclination towards said detector means and the longitudinal axis of said detector means is substantially normal to said photographic film, with the face of said detector means substantially equidistant from said photographic film as said source, measured from the point at which said longitudinal axis of said source intersects said photographic film.
 3. The apparatus of claim 1 wherein said electronic pulse height analyzer means comprises individual pulse height analyzers with associated ratemeters reserved to measurement of said (A) radiation and said (B) radiation individually, adjusting means regulative of the output signal from the (B) radiation measuring one of said associated ratemeters, a difference amplifier receiving as separate inputs the output signals from said ratemeters, and recording means indicating the magnitude of said silver content as a function of the difference between said output signals from said ratemeters.
 4. The apparatus of claim 1 wherein said Americium-241 radioisotope source comprises a plurality of sub-sources arranged in close adjacency to said film symmetrically around said detector means and directed inwardly toward said film to generally collimate radiation emitted toward said film at acute angles of incidence and in non-overlapping impingement on individual areas of said film. 